The gastrointestinal mucosa consists of a simple layer of epithelial cells which are at least partially covered on the side of the intestinal lumen by a viscoelastic layer mainly consisting of glycoconjugates. The epithelial cells synthesize the glycoconjugates present at their surface, which are the intermediates in numerous interactions, in particular with lectins or adhesins, with bacterial toxins, or with antibodies, bacteria, viruses, parasites. These glycoconjugates therefore constitute important intermediates in the relationship between the host and the intestinal flora.
These glycoconjugates are, as their name indicates, glycosylated compounds, that is to say on which are grafted sugar chains which are long to a greater or lesser degree and which may be branched. In the healthy human intestine, these sugars may be galactose (Gal), fucose (Fuc), N-acetylneuraminic acid or sialic acid, N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNac), linked to each other by various bonds. Three of them may be in a terminal position; they are galactose, fucose and sialic acid.
A host and its intestinal microflora function as a complex system in which the microflora has a significant impact on the host. In the case of nonpathogenic or probiotic microorganisms, a symbiotic or cooperative relationship often exists between the host and the microorganisms, the presence of the latter being necessary for good balance and good functioning of the host's intestine. By contrast, the presence of pathogenic microorganisms, which is more rare, may have negative consequences by preventing or reducing the presence of probiotic microorganisms, or even by having a parasitic action which is directly harmful to the host.
The fragile balance between the host and the microflora is directly linked to the intestinal environment and in particular to the quantitative and qualitative presence of the surface glycoconjugates. Indeed, it is known that some microorganisms, which may be probiotic or pathogenic, will be sensitive to certain sugars at the terminal position. The intestinal bacteria can modulate the pattern of glycosylation of the glycoconjugates present at the surface of the intestinal cells, without their mode of action being completely elucidated as will be seen hereinafter. The bacteria can, on the one hand, induce the presence of such and such sugar during glycosylation and, on the other hand, break down the sugars present at the terminal position of a chain, which qualitatively and/or quantitatively modifies the sugars present at the terminal position.
This modulation of the pattern of epithelial cell surface glycosylation causes modification of the intestinal environment, it being possible for the modified environment to promote the establishment of certain microorganisms and/or to limit, or even avoid, the establishment of other microorganisms. Modification of the environment, which may be generated by the microflora, therefore has a direct impact on the balance between the host and the microflora.
It would therefore be highly advantageous to have a model which makes it possible to rapidly and easily select microorganisms according to their action on the pattern of glycosylation. It would thus be possible to identify microorganisms promoting the presence of one sugar or another, and therefore promoting the establishment of beneficial microorganisms such as, for example, probiotics and/or limiting the establishment of pathogenic microorganisms.
Indeed, to the knowledge of the applicant, no microorganisms, in particular lactic acid bacteria, have so far been identified which are capable of finely acting accurately on the glycosylation pattern, and thus of accurately modulating this glycosylation pattern and therefore the implantation of microorganisms.
Such microorganisms modulating the glycosylation pattern could find a use in particular in pharmaceutical or food compositions or food supplements. Indeed, they can make it possible to optimize the functioning of the intestinal cells and the good balance of the microflora.
The document WO 99/29833 describes a novel bacterial strain having certain properties, for example an antimicrobial activity. However, the modalities of its action are not detailed, and in particular the question of the possible action of the bacterium on sugars is never addressed.
Bry L. et al., in “A model of host-microbial interactions in an open mammalian ecosystem”, Science, Vol. 273, pp. 1380–1383, Sep. 6, 1996, studied in vivo (in mice) the influence of Bacteroides thetaiotaomicron on fucosylation. Bacteroides thetaiotaomicron is part of the intestinal flora in humans and in mice. The model used is nevertheless limited to the study of fucosylation, with the exclusion of other types of glycosylation. It is moreover an in vivo model, and therefore longer and more complex to use than an in vitro model.
However, this model made it possible to emit the hypothesis of the existence of a soluble factor secreted by B. thetaiotaomicron, and which is thought to play the role of a signal bringing about the modification of surface glycosylation. This unidentified soluble factor is therefore thought to act without there being direct contact between the bacteria and the target cells.
To the knowledge of the applicant company, no method has been developed to date which makes it possible to select in vitro, easily and rapidly, various microorganisms, in particular various strains of lactic acid bacteria, according to their precise action on the pattern of glycosylation of the intestinal epithelial cells, and more precisely on the modulation or variation of the composition of each of the sugars.
In the context of the present application, the term “lactic acid bacteria” denotes bacteria capable of producing lactic acid, and in particular the nonpathogenic bacteria chosen from the group comprising Streptococcus, Lactobacillus, Lactococcus, Bifidobacterium and Leuconostoc, in particular B. breve, B. longum, L. lactis, S. thermophilus, L. casei, L. helveticus and L. bulgaricus. 